Infrared radiation heating elements



Nov. 22, 1960 P. AlGRAlN INFRARED RADIATION HEATING ELEMENTS Filed Oct. 14, 1958 2,961,628 Patented Nov. 22, 1960 2,961,628 INFRARED RADIATION HEATING ELEMENTS Pierre Aigrain, Paris, France, assiguor to Compagnie Generale de Telegraphie Sans Fil, a corporation of France Filed Oct. 14, 1958, Ser. No. 767,130 Claims priority, application France Oct. 18, 1957 2 Claims. (Cl. 338-269) The present invention relates to infrared heating elements of the type comprising a radiating element combined with a wall having a shape corresponding to that of the element and through which the radiation takes place. This wall must of course be such as to absorb as little infrared radiation energy as possible. It is generally made of silicon.

It is an object of the invention to provide an infraredradiation heating element of improved design and higher efiiciency.

The infrared heating element according to the invention, comprises a wall, shaped as a tube, plate or otherwise, of ceramic material, preferably of alumine, the grains of which are sufiiciently fine for said element to operate in Lord Rayleighs diffusion without absorption zone.

Preferably, the alumina is prepared in such a manner that the size of the elementary crystallites forming the wall is smaller than .1 or even .01 micron, the alumina being preferably obtained from a gamma-type aluminum powder, sintered at a temperature lower than 1200 C. so as to avoid an excessive increase in size of the grains.

According to a preferred embodiment of the invention the alumina wall is enamelled.

The invention will be better understood from the following non-limitative examples of heating device of the invention relating, respectively, to a tube and to a plate of finely grained alumina.

In the appended drawing, the single figure shows a tubular heating device according to the invention.

Example 1 A tube-shaped wall 1 is provided for housing a heating metallic resistance 2, the operating temperature of which is of the order of 1,000 C. Tube 1 is manufactured from a gamma-type alumina powder, the mean grain size of which is approximately .02 to .03 micron.

This powder is mixed with any suitable binder, such as, for instance, a mixture of terpine oil and ceresine, and is then extruded to provide a tubular element. In the course of sintering, which is effected at a temperature comprised between 1,100 and 1,200 C., the gamma-alumina changes into alpha-alumina. The fired tube has a diameter of the order of 15 mm., a wall thickness of about 1 mm. and a length of approximately 500 mm. The tube is then sprayed, for example, by means of a pneumatic gun, with an aqueous solution containing an enamel powder which may, for example, comprise:

Percent Pb At least 40 SiO At least 20 BrO About 10 SnO and A1 About 20 The resulting tube is robust and its mechanical strength is improved by the enamel treatment which in addition renders the tube impervious to gases and vapors so that it may also be employed in an explosive atmosphere.

The tube is transparent to infrared rays, on account of its fine grained structure and the enamel does not impair said transparency. Also owing to the enamel treatment the tube is adapted for protracted storing or resting in a humid atmosphere without absorbing water and consequently without any risk of bursting when put in operation.

Example 2 In this example, the wall is a plate adapted to be placed in front of an infrared energy source comprising a refractory plate of porous ceramic formed with holes, at the surface of which gas combustion takes place. Such a source is commonly known as an infrared radiant panel. The presence of the wall isolates the flame from the ambient air and insures the safety of the system while preventing the projection of particles into the air.

The wall according to the invention is obtained by pressing a mixture of the gamma aluminum powder previously described with a suitable binder, which may be, for instance, a polyvinyl alcohol. After sintering, which is effected at a temperature of between 1,100 and 1,200 C., a rectangular plate of 10 by 20 cm. with a few millimeters thickness is obtained. This plate, which is, preferably, enamelled as in Example 1, is secured in the front of the radiant panel by conventional means.

It is to be understood that above examples are only illustrative of the invention and many variations and modifications may be effected without departing from the scope thereof: in particular, alumina envelopes of different shapes and dimensions may be realized.

The manufacturing process, as described briefly hereinabove, is based on known techniques enabling the production of finely grained elements. There are many variations of such techniques. However they differ notably from the more usual techniques for the manufacturing of industrial coarse grained alumina wherein sintering is effected at much higher temperatures.

A common coarse grained alumina would absorb a substantial portion of the heat emitted by the radiant filament, thereby substantially increasing the temperature thereof. An important loss of heat by convection would result, with a consequent prohibitory decrease of the radiated energy.

Although the ultra fine grained alumina constitutes the substance which is preferred for the manufacture of walls according to the invention, any inorganic powder capable of agglomerating by sintering in fine grained state to form crystallites of less than .1 micron may also be used. This is particularly the case with titanium oxide.

What is claimed is:

l. A heating element comprising an infrared radiation emitting source and a wall transparent to said infrared radiation, said Wall consisting of sintered alumina, the grain size of which is smaller than .1 micron.

2. A heating element comprising an enamelled tube and an electrically resistant body contained in said tube, said tube being of gamma type alumina sintered at a temperature lower than 1,200 0., the grain size of which is smaller than .1 micron.

References Cited in the file of this patent UNITED STATES PATENTS 1,705,300 Maxson Mar. 12, 1929 2,092,815 Shaw Sept. 14, 1937 2,372,212 Lewin Mar. 27, 1945 2,714,096 Suchet July 26, 1955 

1. A HEATING ELEMENT COMPRISING AN INFRARED RADIATION EMITTING SOURCE AND A WALL TRANSPARENT TO SAID INFRARED RADIATION, SAID WALL CONSISTING OF SINTERED ALUMINA, THE GRAIN SIZE OF WHICH IS SMALLER THAN .1 MICRON. 